Flips the switch to build stack_trace_compressor_fuzzer. This was recently
temporarily disabled in rL369079 as it was breaking the sanitizer buildbots.

My diagnosis of the problem is that on clang-only bootstrap builds, we build
gwp_asan before libfuzzer. This causes a discrepancy when the clang driver
attempts to link libclang_rt.fuzzer* as CMake doesn't see a dependency there.

I've (hopefully) fixed the issue by adding a direct dependency for the fuzz
target so CMake can resolve the build order properly. As part of this, the
libFuzzer 'fuzzer' target has to be discovered before the declaration of the
fuzz target.

Thanks for the catch - looks like this resolves the issue. Slight change is that the desire is for the fuzzer to be built with ninja gwp_asan, so the add_dependencies(gwp_asan stack_trace_compressor_fuzzer) is WAI :)

@hctim This breaks the debug build of llvm when the host compiler doesn't build with compiler-rt. I build llvm/clang (release build) as a host compiler, to build llvm,compiler-rt (debug) build. Unfortunately, while building the debug build, the process tries to find the already installed compiler-rt of the host compiler (which is not present). Do you have any guidelines?